Fluidized catalytic cracking (FCC) is one of the major refining methods used in the oil refining industry. The FCC process is employed to crack materials consisting essentially of petroleum-type hydrocarbons to produce products such as fuels for internal combustion engines and heating oils. The cracking process is usually performed in a vertically oriented conduit, or riser including a reactor vessel, that forms part of an FCC system. During the process, hot catalyst particles in an aerated (fluidized) state are typically introduced into a bottom portion of the riser and induced to flow upward. A hydrocarbon feedstock is mixed with steam to become partially fluidized and injected into the catalyst flow as the catalyst travels through the riser, which creates cracking reactions that breakdown the hydrocarbon feedstock into a simpler (lighter) molecular form.
Optimal cracking conditions in an FCC process require a substantially immediate and homogenous mixing of the catalyst and the hydrocarbon feedstock. Such mixing is difficult to achieve, however, and stratified regions of hot catalyst and cold hydrocarbon feedstock typically appear within the catalyst-hydrocarbon flow. Over-cracking and thermal cracking of the hydrocarbon molecules typically occur in the catalyst-rich areas of the flow. Conversely, incomplete cracking of the hydrocarbon molecules usually occurs in hydrocarbon-rich flow regions. These factors can substantially reduce the overall yield of the FCC process. In addition, over-cracking, thermal cracking, and incomplete cracking have undesirable side-effects such as deactivation of the catalyst within the riser due to coke laydown, regeneration of the catalyst within the regenerator due to the combustion of air and residual coke, and the production of excessive amounts of lower-boiling-range gaseous reaction products, e.g., propane and butane gases.
Hence, effective methods for mixing the catalyst and the hydrocarbon feedstock within the reactor vessel are critical to the cracking process, as proper mixing is predicating upon maintaining a constant spray pattern of the hydrocarbon feed stock. The spray pattern is achieved by constraining flow through carefully shaped flow passages. Should the shape of the passages change, the length to diameter ratio (L/D) of the flow passages is altered, which in turn changes the spray pattern. Changes in the geometry and L/D ratio occur most frequently as a result of the erosion of the nozzle material by the moving catalyst of the fluid bed of the riser in which the nozzle is installed.
To ensure proper mixing, spray nozzles have been devised that introduce the hydrocarbon-steam mixture into the upward flowing catalyst; however, the nozzles currently available for use in FCC units have significant limitations. First, the nozzles can generate an uneven spray pattern that reduces liquid contact between the hydrocarbon-steam mixture and the catalyst, which in turn impedes homogeneous mixing leading to over-cracking, thermal cracking and/or incomplete cracking of the hydrocarbon molecules. Second, the nozzles are susceptible to erosion, which significantly alters the nozzle's internal flow passages resulting in altered spray patterns, which can in turn reduce the consistency and overall output of the yield of the FCC process. Similar limitations are present in other refining processes that utilize nozzles to introduce fluids into a mixing vessel, such as reduced crude conversion (RCC) processes.
For example, U.S. Pat. No. 5,553,783 describes a feed distributor nozzle for a fluid catalytic cracker. In highly erosive environments the outside surface can wear and the wear can extend to the interior of the holes as the outside wears (FIGS. 1A and 1B). When the holes change shape by being effectively shortened, they can no longer direct the spray as intended to maintain the desired spray pattern, typically a flat fan spray. With increasing depth of erosion, the flat fan spray becomes ill-defined and, eventually, the spray pattern morphs into an undesirable spray patter such as narrow cone pattern, which significantly reduces the overall efficiency of the FCC and RCC processes.
For the purpose of directing the spray pattern, nozzle covers have been devised that incorporate external bosses (see FIGS. 2A-C, 3A-B and 4A-B); however, none of these configurations are designed to maintain the minimum L/D tolerances required for consistent spray patterns when used in applications that erode the nozzle, such as FCC and RCC applications. For example, in the nozzle shown in FIGS. 2A-C, the bosses have varying L/D ratios designed to direct the spray pattern even with the hole pattern skewed of axis, however the varying L/D ratios make it impossible to maintain a consistent spray pattern if the nozzle erodes. Further, in the nozzles shown in FIGS. 3A-B and 4A-B, the bosses incorporate a “cats-eye” configuration with flow passages having varying L/D ratios, which are designed to provide individual small flat fan spray patterns of varying diameters, but as with the previously described nozzle, is not capable of maintaining a consistent spray pattern if the nozzle erodes.
Therefore, to improve the yield of FCC and other refining processes and reduce maintenance expenses associated with frequent nozzle replacements, there is a need for a spray nozzle that generates a consistently flat spray pattern for improved homogeneous mixing, reduces down stream low pressure zones and eddy currents to minimize catalyst erosion and maximize the flow area of the catalyst, and is capable of maintaining a minimum L/D ratio required as the nozzle erodes to maintain the desired spray pattern for an extended period of time.